A known seat belt retractor spool comprises a spool body with a bore formed therethrough. A ratchet wheel is arranged on one end of the spool body so as to be rotable relative to the body. A torsion bar is arranged within the spool and comprises an elongate bar having a toothed peripheral surface portion formed adjacent each end of the bar. One of the toothed peripheral surface portions is drivingly engaged with a cooperating toothed surface on an inner part of the ratchet wheel. The other toothed peripheral surface is drivingly engaged with a cooperating toothed surface in the bore at the opposite end of the spool from the ratchet wheel. Thus, under normal circumstances the spool rotates with the ratchet. In the event of a crash, the seat belt retractor locking mechanism locks the ratchet wheel, preventing rotation thereof. As the seat belt begins to pay out further due to the forward motion of the occupant, the spool body is caused to rotate. As the ratchet wheel cannot rotate and the spool body rotates, the torsion bar, which is connected between the spool body and ratchet wheel, is twisted. The torsion bar deforms plastically in torsion. The plastic deformation of the torsion bar reduces the seat belt loading on the occupant, which in turn lessens the chance of the occupant receiving an injury from the seat belt.
A disadvantage of the prior art arrangement is that the torsion bar engages the ratchet wheel and spool body by means of a toothed peripheral surface portion. In view of the forces involved, the toothed portion must take up a substantial length of the torsion bar. That reduces the length of the bar able to be given over to plastic torsion and deformation and this reduces the amount of force that can be taken up by the torsion bar.